It has been a usual practice to prevent the occurrence of hydroplaning through an improvement in a draining property of a tire when traveling on a wet road by providing a plurality of main grooves on the tread surface of a tire along the circumferential direction of the tire and, at the same time, providing a number of subgrooves on a rib in a direction traverse to the circumferential direction of the tire to thereby allow the main grooves to communicate with each other.
A gradual increase in the width of the subgroove in going from the tread center region towards the shoulders contributes to an improvement in the draining property of the tire. However, such a tire is poor in other characteristics required in the practical use of a tire, e.g., travelling performances, in e.g., whirling and abrasion resistance and even comes to lose such characteristics, which makes it impossible to put the tire to practical use.
In order to improve the draining property of a tire without sacrificing the driving stability and the abrasion resistance thereof, Japanese Patent Publication No. 53-561 proposes a tire in which the width of the subgrooves is increased in going from the tread center of the tire towards the shoulders thereof and, at the same time, the width and depth of both the subgrooves and the main grooves are specified based on the relationship between the width of the tread and the tread gauge of the tire. In this tire, the draining property can be ensured by an increase in the depth of the grooves provided on the shoulder sections. However, the increase in the depth of the grooves brings about an increase in the movement of the block (or rib) on the shoulder sections, which unfavorably spoils the driving stability during cornering.